But default, Sublime only triggers when you type letters. It shouldn't trigger for numbers or symbols. However, because html tags begin with '<', ST is set to trigger autocomplete when you press <. A selector is how Sublime identifies what type of file you're using and what your scope is within that file. To check your scope, you can press "option+command+p" on OSX or I believe its control+alt+shift+p on windows and linux (but you can check your default keybindings to make sure). The name of all applicable selectors will appear in the statusbar.

As for other uses for this setting, for example, if you use css, you can set autocomplete to trigger on # and have the selector set to source.css. There are many other applications as well.